A liquid crystal display device including a transmissive liquid crystal display panel, which is an example of a flat-screen display device, generally includes an illuminating device that is disposed behind the liquid crystal display panel. The illuminating device includes a fluorescent lamp such as a cold cathode tube that defines a light source, and arranged to adjust the properties of light emitted from the fluorescent lamp and project the light onto a back surface of the liquid crystal display panel. The projected light passes through the liquid crystal display panel and makes an image displayed visible on a front side of the liquid crystal display panel.
Recently, not only the fluorescent lamp but also power-thrifty long-life light-emitting diodes (LEDs) have been used for the light source incorporated in the illuminating device.
In general, when the LEDs are used for long time under a hot environment, the brightness of the LEDs degrades and the longevity of the LEDs is remarkably reduced, so that the LEDs need to be cooled. For example, PTL 1 discloses a configuration with which LEDs are cooled.
FIGS. 11A and 11B are views showing LED substrates, and an illuminating device including the LED substrates, where FIG. 11A is a cross-sectional view showing the LED substrate, and FIG. 11B is a plan view of the illuminating device in which the plurality of the LED substrates are arranged in parallel.
Each LED substrate 201 includes a plurality of LEDs 204 that are disposed on an upper surface of a long flat tube 202 while sandwiching therebetween an insulating layer 203 as shown in FIGS. 11A and 11B. In this case, each flat tube 202 that defines a base material of each LED substrate 201 includes a plurality of channels 202a inside, through which cooling fluid runs as shown in FIG. 11A. Thus, the heat emitted from the LEDs 204 is removed to cool the LEDs 204 by passing the cooling fluid through the channels 202a. 
An illuminating device 200 includes the plurality of LED substrates 201 that are arranged in parallel, and headers 205 and 206 that are communicated with the flat tubes 202 and attached to both the ends of the LED substrates 201 as shown in FIG. 11B, whereby the cooling fluid can be supplied to the plurality of flat tubes 202 at the same time.
In this case, The headers 205 and 206 are connected to a radiator 207 that defines an air-cooled heat exchanger arranged to cool the cooling fluid, a piston-type reciprocating pump 208 arranged to pass the cooling fluid, and a tank 209 arrange to maintain the amount of the cooling fluid via a pipe 210. Thus, cooling of the LEDs 204 is performed by the cooling fluid circulating through the flat tubes 202 of the LED substrates 201 and the radiator 207.